The composition of the normal gastrointestinal flora varies somewhat from individual to individual. Some bacterial species may be carried only transiently, but most are fairly permanent. Some members of the normal flora can become pathogenic if they acquire additional virulence factors (e.g., E. coli) or are introduced into normally sterile sites (e.g., Staphylococcus aureus). Normal flora is general beneficial—for example, the normal flora may prevent pathogenic microrganisms from proliferating in the body (a phenomenon known as colonization resistance), and may also produce essential nutrients (e.g., vitamin K is produced by the gut flora).
The use of antibiotics is ubiquitous among children and adults for bacterial infections, and they are often also prescribed for viral infections. This prolific use has come under criticism for various reasons, most notably for inducing microbial resistance to previously effective antibiotics and rendering them less effective or ineffective against dangerous human pathogens. For example, multidrug-resistant strains of Mycobacterium tuberculosis seriously threaten tuberculosis (TB) control and prevention efforts. Administration of broad-spectrum antibiotics has a profound effect on the normal flora and can result in colonization with antibiotic-resistant organisms. Antibiotic-mediated disruption of the normal flora can lead to fungal infections, such as invasive candidiasis, or to antibiotic-associated colitis caused by Clostridium difficile. 
Members of the genus Clostridium are Gram-positive, spore-forming anaerobic rods. These bacteria are ubiquitous in nature (including the human colon) and are readily found in soil. When stressed, the bacteria produce spores that tolerate extreme conditions that the active bacteria cannot. In their active form, some of these bacteria secrete powerful exotoxins that are responsible for such diseases as tetanus, botulism, and gas gangrene. Clinically important species of Clostridium include C. tetani, C. difficile, C. perfringens and C. botulinum, as well as several others.